The invention relates to hybrid electrolytes having network structures suitable for use in lithium metal batteries.
Solid polymer electrolytes (SPEs) with high conductivity and excellent resistance to lithium dendrite growth are highly desirable for the safe operation of lithium batteries. We herein report the facile one-pot synthesis of nanoparticle-containing cross-linked SPEs based on polyhedral oligomeric silsesquioxanes (POSS) and amine-terminated poly(ethylene glycol) (PEG). Conductivity, mechanical properties, and resistance to dendrite growth of the electrolytes can be tuned by controlling the network structures. Our hybrid SPEs exhibit superior dendrite inhibition even at high current densities. The all-solid-state lithium metal batteries fabricated using our SPEs show excellent cycling stability and rate capability. This hybrid material could significantly improve the performance and safety of lithium batteries. A new class of hybrid electrolytes based on POSS nanoparticles with controlled network structures have been designed and prepared using a facile one-pot reaction. Properties of the SPEs including conductivity, mechanical properties and resistance to lithium dendrites growth can be tuned by changing the cross-linked structures. SPEs with high room temperature ion conductivity (≈0.1 mS/cm) or with high ionic conductivity (>1 mS/cm) combined with high storage modulus (33.6 MPa) at 105° C. have been obtained. The latter one shows superior resistance to lithium dendrites growth compared with reported SPE systems, even under high current densities of 0.5 mA/cm2 and 1.0 mA/cm2. Li/LiFePO4 batteries using POSS-2PEG6K as electrolyte show improved cycling stability and rate capability. The hybrid SPEs therefore are promising for fabricating next generation safe lithium batteries.
Secondary batteries are highly desirable nowadays due to declining fossil resources, increasing demands for clean energy, and rapid expansion of the electronics market. Although the current state-of-the-art lithium ion batteries (LIBs) have been widely used in consumer electronics, because of their relatively low energy density, they cannot meet the needs of applications such as electrical vehicles, autonomous aircrafts, etc. To enhance the energy density, one effective way is to replace graphite (372 mAhg−1) with lithium metal (3860 mAhg−1) as the anode to fabricate a lithium metal battery (LMB).[1] Furthermore, lithium metal can act as the lithium source of the batteries, which enables the use of un-lithiated materials, such as sulfur or air, as the cathodes to fabricate lithium/sulfur or lithium/air batteries with improved energy density.[2, 3] To materialize LMB, the biggest obstacle is the associated safety issues, induced by the uneven deposition of lithium on the lithium metal anode during the charging process. After repeated charge-discharge cycles, this uneven deposition leads to the formation of lithium dendrites, which can connect the two electrodes and short-circuit the cell, causing fire or explosions in some severe cases.[4] Although the growth of the lithium dendrites has been observed using optical microscopy,[5, 6] scanning electron microscopy[7], and hard X-ray microtomography[8], the detailed mechanism of the dendrite formation is still under active research. Chazalviel et al. argued that the anion depletion near the lithium electrode could lead to large electric fields, which in turn, causes dendrites to grow.[9] This model suggests that to prevent anion depletion, electrolytes with high ionic conductivity and low anion mobility are preferred. Alternatively, Monroe and Newman studied interfacial stability in lithium/solid polymer electrolytes (SPEs) systems and proposed that interfacial roughening could be mechanically suppressed if the shear modulus of the separator is about twice that of lithium metal.[10] Following these two frameworks, numerous approaches have been reported in order to achieve LMBs with suppressed lithium dendrite formation. These include tuning the solid electrolyte interface (SEI) with judiciously selected additives,[11-13] using hybrid liquid electrolytes,[14-16] forming alloys of lithium and other metals during electrodeposition,[17, 18] and employing sophisticated SPEs.[19-25] However, despite those extensive efforts, the inhibition of lithium dendrite growth in high current densities (>0.5 mA/cm2) still remains a challenging task, and is considered as a roadblock for LMBs to reach the market place.
Among all the methods mentioned above, SPEs are of particular interest because in addition to their improved dendrite resistance, SPEs also avoid the presence of flammable organic solvents and therefore directly leads to safer all-solid-state batteries. Poly(ethylene oxide) (PEO), which has strong lithium ion solvating ability and high dielectric constant, has been extensively used for SPE systems. Mainly five types of PEO-containing SPEs show promising properties, including cross-linked networks,[22] nanoparticle-containing hybrid SPEs,[26] block/grafted copolymers,[19-21] and SPEs via polymerization-induced phase separation[27] and holographic polymerization.[28, 29] To date, cross-linked SPEs based on polyethylene,[22] polyurethane,[30] polysiloxane,[31, 32] polyacrylate,[33] and polyphosphazene,[34] have been reported. In particular, Khurana et al. recently reported an elegant cross-linked polyethylene/PEO SPE with excellent lithium dendrite growth resistance (Cd value of 1790 C/cm2 for the lithium symmetric cell at a current density of 0.26 mA/cm2).[22] For PEO-based hybrid SPEs, nanoparticles, such as TiO2,[19] SiO2,[23, 24] Al2O3,[35] and ZrO2,[36] have been introduced to PEO homopolymers or block copolymers to obtain enhanced conductivity and resistance to the growth of lithium dendrites.